This application claims priority to an application entitled xe2x80x9cPower Control Apparatus and Method for Inter-Frequency Handoff in CDMA Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Feb. 13, 1999 and assigned Ser. No. 99-5263, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a power control apparatus and method for a mobile communication system, and in particular, to an apparatus and method for performing power control during inter-frequency hard handoff.
2. Description of the Related Art
In general, many mobile communication systems use a Code Division Multiple Access (CDMA) technique. For power control in the reverse link of a CDMA system, the base station measures the strength of the pilot signal transmitted from a mobile station and compares the measured value with a predetermined power control threshold. When the measured pilot signal strength is lower than the threshold, the base station generates a power-up command, and otherwise, when the measured pilot signal strength is higher than the threshold, the base station generates a power-down command. The base station transmits the power control command back to the mobile station in order to control transmission power of the mobile station transmitter. Generally, the power control threshold is set to a predetermined step value according to whether or not there exists a frame error after channel decoding.
In a system where the mobile station and the base station exchange data at certain frequencies, there are times when the frequency being used needs to be changed. The process for changing the frequency is referred to as inter-frequency handoff. Cases of inter-frequency handoff include (1) when the mobile station changes to a frequency of a base station other than the present base station in service, (2) when the mobile station changes from the present frequency to another frequency because of a lack of system capacity for the present frequency or the present frequency has a bad channel environment, and (3) when the mobile station changes from one communication technique to another communication technique.
As one of the initial steps in performing inter-frequency handoff, the base station sends a message commanding the mobile station to measure the channel condition of the new frequency. Upon receipt of the message, the mobile station changes (or shifts) the transmission/reception (Tx/Rx) frequency to the other frequency, measures the strength of the received signal at the changed frequency (or measures Ec/Ior, the energy to interference ratio of the received signal), and then returns to the original frequency. This may occur within a specific frame or over two frames. When measuring the channel condition of the different frequency, transmission of the signal on the original frequency will be interrupted for a moment due to the change of the frequency. This is because the mobile station, using a single frequency synthesizer, simultaneously changes the Tx frequency and the Rx frequency. In order to compensate for the energy loss due to the interruption in signal transmission at one portion within a frame, the mobile station increases transmission power at the other portion within the frame. Here, losing a certain duration within the frame in order to measure the channel condition of the different frequency is referred to as slotted mode (or compressed mode). In slotted mode, the data rate is increased by controlling the spreading factor (SF) at the remaining data transmission duration within the frame. This allows the transmission of data, which would have been transmitted during the non-transmission duration or interruption, during the remaining data transmission duration. In this case, the transmission power at the transmission duration is increased to compensate for the energy loss at the non-transmission duration.
Thus, because the transmission power temporarily increases at the portions of the frame other than the duration where the mobile station measures the channel condition of a target frequency, the power control performed between the base station and the mobile station needs to be changed. The power control process will become more apparent from the following description. Herein, the first frequency refers to the frequency at which the mobile station transmits presently, and the second frequency refers to the target frequency to which the mobile station will move when it performs the inter-frequency handoff.
The base station instructs the mobile station to measure the channel condition of the second frequency, and the mobile station interrupts transmission during a specific part of the frame in order to measure the channel condition of the second frequency according to the base station""s instruction. However, the base station cannot know the exact starting point and duration of the interrupted transmission. That is, because the mobile station measures the channel condition of the second frequency unilaterally without forewarning the base station as to the starting point and duration of the interrupted transmission, the base station does not know the exact transmission-off or interruption duration. The base station will receive all the frame data, including the pure interference signal received during the transmission-off duration, and transmit the entire frame to the channel decoder. Therefore, a receiver of the base station will decode the pure interference signal, mistaking the interference signal for traffic data. In this case, the error rate of the decoded traffic data increases as compared with the case where the portion corresponding to the interference signal is set to xe2x80x980xe2x80x99 indicating a undecided value. The undecided value to is set to xe2x80x980xe2x80x99 on the assumption that the transmitted values are +1 and xe2x88x921 (xe2x80x980xe2x80x99 having been binary-mapped to xe2x80x98+1 xe2x80x99 and xe2x80x981xe2x80x99 to xe2x80x98xe2x88x921xe2x80x99 before transmission). To prevent this, the base station and the mobile station exchange information about the exact starting point and duration (tsearch [ms]) of the transmission-off duration through signaling (hereinafter, the information exchanged between the base station and the mobile station will be referred to as inter-frequency hard handoff signaling). The mobile station changes from the first frequency to the second frequency according to the inter-frequency hard handoff signaling, and measures the channel condition of the second frequency.
FIG. 1 shows the variation in mobile station transmission power during the initial stages of an inter-frequency handoff according to the prior art. The term xe2x80x9cregular power controlxe2x80x9d as used herein means that the mobile (or base station) increases or decreases the transmission power by a predetermined step size according to a power control command transmitted from the base station (or mobile station). For example, the predetermined step size can be set to xc2x10.25, 0.5 1.0 or 2.0 dB. Power control is performed on a power control group (PCG) unit basis, wherein the frame length is a multiple of the power control group.
Referring to FIG. 1, the mobile station transmits a signal by performing regular power control until time A, when it increases the transmission power by xcex94search [dB ], in order to compensate for the loss of the transmission signal during the transmission-off duration tsearch [ms] when the channel condition of the second frequency is measured. During frame durations A-D, the base station maintains the previous threshold for the received pilot signal power even though there is a transmission-off duration during this time period. Therefore, when the mobile station increases the transmission power at time A, the pilot signal power received at the base station after time A is higher than the threshold, so that the base station continuously sends a power-down command to the mobile station. If the mobile station obeyed the power-down command, it would be meaningless to increase the transmission power in the first place. Therefore, the mobile station disregards the power-down command and obeys only the power-up command from the base station during the frame when the channel condition of the second frequency is measured. Between time B and time C, the mobile station changes its frequency from the first frequency to the second frequency in order to measure the channel condition of the second frequency by measuring the traffic signal power or the pilot signal power received at the second frequency. At time C, the mobile station changes its frequency from the second frequency back to the first frequency, and transmits the transmission signal at the first frequency while still disregarding the power-down command. At time D, when the frame ends, the mobile station decreases the transmission power by xcex94search [dB] and then performs regular power control.
FIG. 2 shows the variation in transmission power of a base station during the initial stages of an inter-frequency handoff according to the prior art. Referring to FIG. 2, the transmission power of the base station undergoes regular power control until time A. Before the frame containing the signal transmission interruption is transmitted, the mobile station increases the power control threshold (or target Eb/No) of the mobile station by xcex94target [dB]. This increased power control threshold results in the mobile station transmitting power-up commands to the base station at point A, before the frame starts at point B. The additional increment xcex94target [dB] to the power control threshold during the compressed mode of the mobile station can vary depending on the time duration tsearch and can be determined through experiments. Since the power control threshold of the mobile station has increased, the mobile station continuously sends the base station a power-up command from the time A on, resulting in the base station increasing the transmission power, as shown from time A to time C. The power-up command may be continuously transmitted until the frequency is shifted to the second frequency at time C. At time D, where the frequency switch ends, the base station returns to the original frequency and continues the interrupted data transmission using the first frequency. At time E, when the frame ends, the mobile station decreases the power control threshold by xcex94target [dB].
In the existing power control method described with reference to FIGS. 1 and 2, the mobile station takes the initiative in measuring the channel condition of the second frequency for the inter-frequency handoff. The power control is performed by changing the transmission power of the mobile station as shown in FIG. 1, or by changing the power control threshold of the mobile station as shown in FIG. 2.
The existing method for measuring the channel condition of the second frequency for the inter-frequency handoff has several disadvantages. In the power control method of FIG. 1, in which the mobile station disregards the power-down command transmitted from the base station, if the mobile station experiences deep fading, the mobile station obeys the power-up command and increases its transmit power. Thereafter the channel condition become better, the mobile station disregards the power-down command, thereby resulting in a waste of the transmission power. In addition, data transmission at higher transmission power increases interference in the reverse link, thus causing a decrease in the capacity of the reverse link. Further, in the power control method of FIG. 2, when the power control step size is small, the mobile station needs to transmit the power-up command for a quite long time in order to increase the transmission power to the desired value. In this case, there is a portion in a previous frame where the regular power control cannot be performed, even though that frame does not have an interrupted transmission duration.
Therefore, there is a need for a method of using regular power control without disregarding the power-down command during the initial stages of the inter-frequency handoff, when power control occurs, as well as a method for rapidly controlling transmission power of the base station by a desired value.
It is, therefore, an object of the present invention to provide a device and method for performing regular power control for a frame duration, which includes a target frequency measuring duration, during an inter-frequency handoff in a mobile communication system.
It is another object of the present invention to provide a device and method for rapidly controlling frame transmission power by a desired value during an inter-frequency handoff in a mobile communication system.
It is further another object of the present invention to provide a device and method for performing regular power control for a frame duration, which includes a transmission-off duration, during an inter-frequency handoff in a mobile communication system.
To achieve the above object, a power control method for a mobile station having transmission frames is disclosed, at least one transmission frame including a transmission duration when data is transmitted on a first frequency, and a transmission-off duration when no data is transmitted on the first frequency and a second frequency is searched in order to perform an inter-frequency handoff to the second frequency, said transmission duration having increased transmission power in order to compensate for a loss of transmission power on the first frequency during the transmission-off duration, the method comprising the steps of: setting, in a base station, a power control threshold depending on a length of the transmission-off duration; and receiving, in the base station, transmission power-increased data; comparing the power of the received signal with the power control threshold; generating a power-up command when the power control threshold is higher than the received signal power; and generating a power-down command when the power control threshold is lower than the received signal power.